Modern wind turbines of multi-Megawatt class have wind turbine blades that are aerodynamically optimized for high efficiency. Maintaining high efficiency is central to the profitability of wind turbine operation, since competition amongst electrical power producers has brought the profit margins down into single digits, in percent, and changes in efficiency that might be overlooked by the casual observer are indeed strongly felt in the profit and loss statements.
The control logic for each wind turbine plays a big role in the total energy generated. Another strong influence on efficiency comes from the blade's aerodynamics. The aerodynamics influence the lift force, hence the power producing force, that the blade generates, as well as the drag force that reduces the power generating ability of the turbine.
During operation, the blades move rapidly through the air and are, thereby, subject to the accumulation of dirt or organic material on their surface due to collisions between the blades and particulates (e.g., salt, dirt), or insects in the air.
Blades can be mass produced with a given shape, or can be tailored during design for maximum performance at a given site. In tailoring, the design is adopted to the anticipated blade surface state at the given site, such as very smooth blade surface (without roughness) for a site exposed predominantly to clean air, or with some characteristic roughness for a site exposed to dirt or frequent impacts with insects.
When, during operation, the blade surface deviates from the surface condition assumed during the design process, a detrimental loss of aerodynamic efficiency, and, therefore, produced energy, occurs. It is desirable, therefore, to measure, or assess, the surface condition of the blades at all times to determine the rate of lost energy, hence the rate of lost revenue. Based on this information, the optimum corrective action can be taken, such as, for example, planning a surface cleaning maintenance at the best possible time (a decision that includes comparing the accumulated cost of lost energy production with the cost of a scheduled or an unscheduled maintenance activity).
Additionally to material accumulation on the blade surface, the leading edge of the blade may undergo erosion during operation, due to the accumulated effect of energetic collisions with particulates in the air. Erosion is an extreme case of deterioration of the blade surface condition, characterized by a loss of surface material.
Knowledge of the clean or dirty state of the blade's surface can be used favorably by the turbine operator, by (a) adopting the control strategy to maximize the efficiency with the current clean/dirty state of the blade surface, as discussed in EP 2 818 698 A1, and (b) by scheduling at the best time the blade cleaning, so as to minimize power loss during stand-still.
Maintaining the blades with a clean surface condition, thus, is central to maintaining high wind turbine efficiency. Currently, the surface condition of blades is assessed by simple visual inspection, either by a ground based observer or, less often, by a service person repelling along the blade. This surface inspection is very infrequent, typically occurring during a selected maintenance operation, thereby allowing a wind turbine to operate with dirty blades for extended periods of time before remedial action is taken. Unfortunately, there are no products or systems commercially available today for continuously monitoring a blade surface condition. It is highly desirable, thus, to have a system for assessing the surface condition of the blades that operates continuously, and that can provide a numerical measure of the level of dirtiness, so that both adaptations can be made in the wind turbine control logic, and blade-cleaning scheduling can be optimally planned to minimize lost revenue.
There are numerous products for detecting blade deflections, and blade material strain. For example, National Renewable Energy Labs Technical Report NREL/TP-500-39253 (January 2006) teaches an optical system for measuring blade deflection at selected spanwise locations, while WO 2008/020242 A2 discloses a fiber-optic based system with Bragg gratings for measuring wind turbine material strains, and US 2011/0150649 A1 teaches the use of an accelerometer located in the outward portion of the blade and a procedure to obtain blade deflections from its signal. These systems perform blade-deformation measurements on a continuous basis, without the need of a human-in-the-loop in the process of evaluating the generated signals.
It is further highly desirable to have a system for assessing the surface condition of the blades that uses technology free of human-in-the-loop requirements for signal analysis, hence is able to provide a signal indicative of blade surface condition on a continuous, automatic, basis.